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WASHINGTON - One of President Donald Trump's central campaign promises was to build a "big, beautiful, powerful wall" along the border with Mexico to keep out undocumented immigrants and drug smugglers. But according to John F. Kelly, the homeland security secretary, this wall will not actually stretch across the entire border.

"It's unlikely that we will build a wall or physical barrier from sea to shining sea," Kelly told the Senate Homeland Security Committee Wednesday. A wall will instead be built where it "makes sense," while other portions of the border might include high-tech fencing or other "technology."

Kelly's statement was the clearest admission from the administration that the wall might not be built along the full 2,200-mile length of the Southwest border, which already has hundreds of miles of fencing. Kelly said he is committed to erecting the structure "where the men and women say we should put it," and is looking at numerous variations of Trump's promised wall.

The homeland security secretary also said there was "no way" he could provide an estimate for the wall's cost, not knowing what materials it will be made of, how high it will be, or other details about the structure's design. Previous cost estimates from various government sources have ranged from $12 billion to $21 billion, according to the Associated Press.

Tuesday marked the deadline for the first bids for border wall prototypes, which will be built and tested in San Diego, The Washington Post reported. U.S. Customs and Border Protection declined to identify bidders or say how many there were, which is common in government contracting. The federal government expects to announce around June 1 which companies will be hired to build prototypes.

Early requests for proposals offered a glimpse of possible prototypes - such as one that called for reinforced concrete and another that envisioned durable see-through material. Other companies released their plans Tuesday, with one bidder proposing a border wall with solar panels and another suggesting building a wall large enough for a deck that would offer tourists views of the desert, according to the Associated Press.

Prototypes are required to withstand "for a minimum of 1 hour" efforts to breach it by punching, using a sledgehammer, or a "car jack, pick axe, chisel, battery operated impact tools, battery operated cutting tools, Oxy/acetylene torch or other similar hand-held tools," according to requests for proposal. The requests also said the wall must have anti-climbing devices and mechanisms to prevent tunneling under it to a depth of six feet.

During his testimony to the Senate Homeland Security Committee, Kelly also indicated a reversal of previous statements he made concerning families crossing the border illegally. On March 6, Kelly said on CNN that in order to deter illegal immigration, the department was considering separating children from parents caught crossing the border.

This practice would result in detention for the parent while any accompanying children would be placed in the care of the government or sent to live with any relatives in the United States.

"I would do almost anything to deter the people from Central America getting on this very, very dangerous network that brings them up from Mexico," Kelly told CNN's Wolf Blitzer at the time.

But Wednesday, Kelly said such a separation of families would only take place on rare occasions, "if the situation at the time requires it," such as if the mother is sick or addicted to drugs.

Kelly touted a recent drop in numbers of people crossing the border - March marked the second straight decline in arrests at the border, and the fewest arrests in a month in at least 17 years. Customs and Border Protection said 12,193 people were caught trying to enter the United States illegally across the Mexican border over the course of the month. In December, that number was at 43,250.

Kelly said that decline won't continue unless his agency gets the resources needed to secure the border.

"All we know is that physical barriers do work if they are put in the right places," Kelly said.

In response to Kelly's comments, Sen. Claire McCaskill, D-Mo., the ranking Democrat on the committee, urged the agency, and Trump, to stop claiming Mexico will be paying for a border wall "from sea to shining sea."

"It's embarrassing," said McCaskill. "It's not going to happen."

"Everybody in Congress knows it's not going to happen," she added. "It appears the only person who won't say it out loud is the president of the United States."